4.88 or 5.13 rpm at 70

[kennyglm]

i am looking to upgrade my rears to either 4.88 or 5.13 and was wondering if theres a way to see what my rpm would be at 70 mph with my automatic 8 speed tranny i have 37x13.50x20 tires. thanks

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[OHJeeper]

https://purperformance.com/p-29669-rpm-calculator.html

 

[kennyglm]

https://purperformance.com/p-29669-rpm-calculator.html

i have no idea what the transmision gear ratio is . how do i find out ?

 

[NachoRuby]

i have no idea what the transmision gear ratio is . how do i find out ?

https://www.google.com/url?sa=t&sou...QQFnoECCUQAQ&usg=AOvVaw3Cvr6JzkmGYCPSnJ4LYntJ

 

[Escape.idiocracy]

https://www.google.com/url?sa=t&sou...QQFnoECCUQAQ&usg=AOvVaw3Cvr6JzkmGYCPSnJ4LYntJ

for this being an FCA page I think it’s weird that they only list 4.10 for axle ratio with the NV241OR

 

[Escape.idiocracy]

For those that don’t want to click through, but are still interested in the data.
Stock diesel rubicon- or 3.6 non rubicon.

Stock rubicon

Factory geared diesel 37” tires- non rubicon 3.6 factory gears

4.56’s & 37’s

4.88 & 37’s

5.13 & 37’s

 

[OHJeeper]

for this being an FCA page I think it’s weird that they only list 4.10 for axle ratio with the NV241OR

Isn't that because the Rock-Trac is only available on the Rubicon and the Rubicon only has 4.10 from the factory?

 

[NachoRuby]

Isn't that because the Rock-Trac is only available on the Rubicon and the Rubicon only has 4.10 from the factory?

The diesel is st 3.73, even on Rubicon.

 

[vst]

I have 5.13's with 37x12.50 and at 70 mph im just under 2200 rpm love the way the gears feel has plenty of power in my opinion

 

[kennyglm]

I have 5.13's with 37x12.50 and at 70 mph im just under 2200 rpm love the way the gears feel has plenty of power in my opinion

thank you for the honest answer. how did it affect your gas mileage ?

 

[taintedsaint]

To that end, I'm running 4.88 with true 37s. At 70 I'm just a tick over 2000 rpm.

 

[vst]

thank you for the honest answer. how did it affect your gas mileage ?

I bought the jeep with the setup already so im not sure about stock gas milage but I currently get around 16 mpg around town and 17-19 on the highway if I don't go above 70 mph

 

[OHJeeper]

The diesel is st 3.73, even on Rubicon.

huh... I never knew that!

 

[phunktion]

Spreadsheet linked in this thread is the best I've found on comparing gear ratios and tire size.

With your inputs, here's stock on the left, 4.88s on the right. 70MPH would be 2147RPM in 8th (or in that vicinity) using the tire effective diameter of 35.9" from the calculator. If you know your actual diameter is higher, just tweak the tire size in the calculator to bring it to where you know it to be.

 

[Snapper692]

I'm about 2300rpm at 70mph, 2550rpm at 80mph.

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